Two-step chemical process of fiber liberation



Patented Nov. 14, 1933 UNITED STATES PATENT- OFFICE TWO-STEP CHEMICAL PROCESS OF l IBER LIBERATION George A. Richter, Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporation of Maine,

No Drawing. Application February 15, 1930 Serial No. 428,838

7 Claims.

as used as a raw material in the papermaking and cellulose derivative industries.

An object of the present invention is to make possible the utilization of highly resinous species of woods, such as long leaf yellow pine, in the production of high grade pulps which are characterized by physical characteristics, including strength and tear resistance, approximating those of kraft pulp, but which possess the important advantage over kraft pulp in that they are more easily bleachable. Another object of the present invention is to produce such pulp in a practical period of time and without the use of temperatures higher than'those ordinarily employed in the soda or kraft process.

In accordance with the process of the present invention, the wood employed as a raw material is initially cooked short of fiber liberation in a suitable alkaline liquor, preferably in a kraft liquor, whereupon fiber liberation is completed in a neutral or only slightly alkaline sulphite liquor. Such a two-step. chemical process of fiber liberation may be practised to advantage upon various types of woods but is especially advantageous in the case of woods of high resin content, as the alkaline liquor used inthe initial cooking operation saponifies most of the resins present in the wood, whereupon fiber liberation into a high grade bleachable product may be readily efiected in a neutral or slightly alkaline sulphite liquor, because of the reactivity of the sulphite with residual lignin present in the wood. A significant advantage of the process of the present invention is that both cooking steps may be accomplished in a single alkaline digester, as neither cooking liquor is acid. After the initial cooking operation, the spent alkaline liquor may be discharged from the digester and, while the wood is still hot, the fresh sulphite liquor may be run into the digester, preferably in heated condition, so as to avoid cooling of the wood.

In carrying out the initial step of the present process, a liquor of much'lower alkalinity than that used in the usual soda or kraft process may be employed. For instance, the chipped wood may be placed in an alkaline ,digester together with an alkaline liquor containing 0.5 to 1.0 pounds of active alkali per cubic foot, which represents an alkali content of about half that present in the usual soda or kraft process. While the alkali may be present in the form of either caustic soda or sodium sulphide, it is preferable to use a mixture of both these alkalies, about 30% to 40% being present as sodium sulphide, as in the usual kraft process. The charge of chips and liquor is confined in the digester and is cooked at a temperature of, say, 300 to 350 F. for about two to four hours, at the end of which time the chips are brown and somewhat softer than the original wood, Whereas the cooking liquor is dark brown. The chips are separated from the spent liquor by opening the blow pipe valve at the bottom of the digester and permitting free liquor to drain from the chips. The chips are preferably'washed, but this step may be omitted, if desired. The blow pipe valve is then closed and a suitable sulphite liquor, preferably in hot condition, is added to the digester, whereupon the digester contents are confined. The liquor should contain about 5% to 10% of a soluble sulphite, preferably sodium sulphite, so that fiber liberation may be effected in a reasonable period of time without requiring excessive temperatures. The digester contents are ,heated to temperatures of from 300 to 350 F., and cooking at these temperatures is continued from two to six hours to accomplish the desired completion of fiber liberation. The resulting pulp is then washed free of spent liquor and may be bleached or otherwise chemically treated before being used in the manufacture of paper or other ultimate products.

When woods of high resin content, such as long leaf yellow pine or other conifers, are processed as hereinbefore described, the spent liquor resulting from the initial cooking operation contains a large amount of resin soap resulting from the saponification of most of the resin content of the wood. The cooked wood, being relatively free from resins, is much more readily reacted upon by the sodium sulphite solution to produce a pulp than is the case whenno initial treatment such as described is efiected. When the process is carried out under the conditions hereinbefore described, the product not only has strength and tear resistance practically as good as kraft pulp, but is more readily bleachable than kraft pulp, doubtless as a result of the high reactivity of sodium sulphite upon residual lignin present in the wood after. the initial cooking operation. If the initial cooking operation is carried out with weaker liquors, or at lower temperatures than those prescribed in the example, the resulting pulp has a tear resistance lower than that of kraft pulp, and its other characteristics approach those of pulps produced by cooking only in sodium sulphite liquors.

If desired, the wood chips may first be cooked in the weak alkaline solution at the desired temperature for the requisite period of time, whereupon the desired amount of sodium sulphite liquor may be pumped into the digester without draining the initial liquor from the chips, and cooking in the sulphite liquor may then be effected to complete fiber liberation. The sodium sulphite liquor may be added in the form of a highly concentrated solution, in order that it may be accommodated by the free space in the digester. Such a procedure is advantageous, especially when the resulting pulp is not to be bleached, as the heat content of the liquor is conserved, the process simplified, and all the sodium constituent recovered from a single waste liquor of comparatively high concentration in sodium and organic constituents. Even if all the active alkali used in the first cooking step is not consumed before the addition of the sodium sulphite, it does not interfere with the liberation of fiber accomplished by the second cooking step. The withdrawal of the spent alkaline liquor at the end of the first cooking step improves the bleachability of the final pulp, but in those cases where bleachability of the resulting pulp isnot an important factor, it is possible, by adding the sodium sulphite to the spent alkaline cooking liquor associated with the wood chips after the initial cooking step, to effect a saving of time, heat, and labor.

The sulphite liquor used for the second cooking operation should be non-acid, by which is meant that it may be neutral or slightly alkaline. As already pointed out, such a liquor not only makes possible the production of a high grade product, but may be used satisfactorily in the usual alkaline digester, so that there is no necessity for removing the chips :from the digester after the initial digestion, a procedure which would be difiicult and expensive in actual practice.

I claim:

1. A process which comprises cooking raw cellulosic material short of fiber liberation in an alkaline, sodium base liquor, separating the liquor from the cooked material, and then cooking the material to complete fiber liberation in a substantion, removing only the cooking liquor from the.

digester, introducing a substantially neutral solution of essentially only a sulphite thereinto, and cooking the material in said solution to complete fiber liberation.

3. A process of producing pulp from a resinous- Wood, which comprises initially cooking chips of such wood in an alkaline, sodium base cooking liquor to effect a saponification of most of the resin content while maintaining the wood in chip form, and then cooking the chips in a substantially neutral solution of essentially only sodium sulphite to complete fiber liberation.

l. A process which comprises cooking chipped wood in a sodium base, alkaline liquor short of fiber liberation, and then cooking in a substantially neutral solution of essentially only sodium sulphite to complete fiber liberation.

5. A process which comprises cooking chipped wood in-a kraft cooking liquor short of fiber liberation, and then cooking in a substantially neutral solution of essentially only sodium sulphite to complete fiber liberation.

6. A process which comprises cooking chipped wood short of fiber liberation in an alkaline sodium base liquor made up essentially of caustic soda and sodium sulphide with the caustic soda somewhat preponderating in amount, and then cooking to complete fiber liberation in a substantially neutral solution of essentially only sodium sulphite.

'7. A process which comprises cooking chipped wood short of fiber liberation in an alkaline sodium base liquor made up essentially of caustic soda and sodium sulphide with the caustic soda somewhat preponderating in amount, and then cooking in a substantially neutral solution of essentially only sodium sulphite of about 5% to 10% strength at about 300 to 350 F., for about two to six hours, thereby producing a completely liberated wood pulp of easy bleachability.

GEORGE A. RICHTER. 

